Quads were first allowed in the senior men's short program in the 1998-99 season, as the solo jump preceded by steps. I think it was in 2000-01 that quads were first allowed in the jump combination.

Since 1991, it has not been allowed to do the same jump for two different jump elements in the SP. (I.e., the only way you're allowed to repeat a jump in the SP is if you do a 3toe-3toe or 3loop-3loop combo . . . or double-double combo in juniors.)

With that restriction only, it would theoretically be legal to do a 4salchow combination and a solo 4toe, or vice versa.

Element c) is "One jump combination consisting of a double jump and a triple jump or two triple jumps or a quadruple jump and a double jump and a triple jump."

The clarification for element b) states that "For Senior Men any triple jump or quadruple jump is permitted, however when a quadruple jump is executed in c) a quadruple jump cannot be included again as a solo jump."

The clarification for c) states "For Senior Men when a quadruple jump is executed in b), a quadruple jump cannot be included again in the jump combination."

Someday, if enough skaters are successfully executing two or more different quads in their long programs on a regular basis, they may change the wording so that it would be legal to do two different quads in the short program. For now, though, it's not allowed.

See pp. 86-92 for the short program requirements; the relevant passages are on pp. 87 and 89.

If you look up video of Yagudin or Plushenko's short programs from the 2000 worlds, you'll see examples of the quad as the solo jump out of (minimal) steps. I can't recall if Stojko, Goebel or Weiss had the same layout of jump elements that year (triple axel-triple toe, quad toe and double axel).

The rule then was a double or triple axel, a jump combo of triple-double or triple-triple, and a solo triple or quad. Skaters who chose to do the quad, usually made their combo a triple axel-triple toe. Because they were barred from repeating the triple axel as the specified axel jump, they had to then do a double instead.

Most of the Chinese men did this and likely Honda as well. Most other skaters did four triples instead.